Denny Stohr, M.Sc.

Büro: S3/20  Raum 221
Rundeturmstr. 10
64283 Darmstadt, Germany

Tel.: +49 6151 16-21024
Mail: denny.stohr(a)cs.tu-darmstadt.de

Research Interests:

  • Quality of Service / Quality of Experience for HTTP Adaptive Streaming
  • Network monitoring
  • Distributed systems

Curriculum Vitae:

Denny Stohr studied Computer Science and Economics since september 2008 at the University of Mannheim. He graduated with Master of Science in Computer Science in 2013 with his thesis "Techniques for Robust, Digital Watermarking for 3D Videos". During his studies he was interested in communication networks, operations research and computer vision. He worked as a research assistant for the University of Mannheim at BWL.POOL. He is currently a PhD student and works as a member of the DFG Collaborative Excellence Cluster MAKI.

We are always looking for talented Hiwis and students interested in a thesis.

Publications:

[SFF+16] Denny Stohr, Alexander Frömmgen, Jan Fornoff, Michael Zink, Alejandro Buchmann, and Wolfgang Effelsberg. QoE Analysis of DASH Cross-Layer Dependencies by Extensive Network Emulation. In Internet-QoE (SIGCOMM), Florianopolis, 2016. ACM.
With the rising importance of video streaming in the Internet, dynamic adaptive streaming over HTTP (DASH) has been established as a key technology for video delivery. Yet, variable network conditions often result in a limited quality of experience (QoE)—with the interrelation of cross-layer network factors and DASH mechanisms widely unexplored. To understand the complex dependencies between DASH configurations and network conditions, we propose a systematic extensive large-scale emulation approach with state-of- the-art QoE metrics. Using this approach with a real DASH player in Mininet, we emulated more than 10, 000 combinations of static and dynamic network conditions and DASH configurations to derive their QoE. The obtained results show that no single DASH configuration provides the highest achievable QoE. Depending on the network conditions, specific combinations of the TCP congestion control, segment sizes and the DASH adaptation algorithm provide higher QoE—showing the possibility of performance improvements in practice. Furthermore, the ex- tensive emulations show a linear relation between delay, loss and QoE that is mostly independent of bandwidth.

[FSF+16b] Alexander Frömmgen, Denny Stohr, Jan Fornoff, Wolfgang Effelsberg, and Alejandro Buchmann. Capture and Replay : Reproducible Network Experiments in Mininet. SIGCOMM, pages 2–4, 2016. [ DOI ]
Network simulations and emulations are widely used in the networking community. The network emulator Mininet recently gained popularity, as it allows running real Linux applications on top of an emulated network. The specification of the network includes the topology as well as static bandwidth, latency, and packet drops probability parameters. Even though evaluations with static parameters provide useful insights, real world measurements show dynamically changing bandwidths, posing special challenges that need to be addressed when conducting networking research. In this demo, we capture bandwidth traces in the wild and reproducibly replay these traces in Mininet. Our capture and replay infrastructure consists of a Mininet extension for replaying bandwidth traces, and a measurement Android app, as well as a graphical repository for bandwidth traces. We exemplary demonstrate this toolchain for reproducible DASH and Multipath TCP experiments.

[WSSE16] Stefan Wilk, Dominik Schreiber, Denny Stohr, and Wolfgang Effelsberg. On the effectiveness of video prefetching relying on recommender systems for mobile devices. In 2016 13th IEEE Annu. Consum. Commun. Netw. Conf., pages 429–434. IEEE, jan 2016. [ DOI | http ]
High quality video streaming over cellular networks is expensive for users and often limited by data caps. Offloading the traffic over other cheap wireless networks, e.g. WLAN, would be beneficial however they are often not available for mobile streaming sessions. A potential solution is to load video to a mo- bile device before the user accesses it, which is called prefetching. Prefetching requires indicators to perform a prediction what a single user is going to watch in the future. This prediction determines which videos are loaded to the users' mobile device. We leverage recommender systems of the video sharing site YouTube to design and evaluate mobile prefetching algorithms. A user study has been performed over a month in which users' YouTube accesses were supported by our prefetching algorithms which allowed for significant reductions in data usage on mobile networks

[FSF+16a] Alexander Frömmgen, Denny Stohr, Jan Fornoff, Wolfgang Effelsberg, and Alejandro Buchmann. Capture and Replay. In Proc. 2016 Conf. ACM SIGCOMM 2016 Conf. - SIGCOMM '16, pages 621–622, New York, New York, USA, 2016. ACM Press. [ DOI | http ]
Network emulations are widely used in the network- ing community. The network emulator Mininet recently gained popularity, as it allows running real Linux applica- tions on top of an emulated network. The specification of the network includes the topology as well as static band- width, latency, and packet drops probability parameters. Even though evaluations with static parameters provide useful insights, real world measurements show dynam- ically changing bandwidths, posing special challenges that need to be addressed in network research. In this demo, we capture bandwidth traces in the wild and reproducibly replay these traces in Mininet. Our capture and replay infrastructure consists of a Mininet extension for replaying bandwidth traces, a measurement Android app, as well as a graphical repository for band- width traces. We exemplary demonstrate this toolchain for reproducible DASH andMultipath TCP experiments.

[WE16] Stefan Wilk and Wolfgang Effelsberg. The content-aware video adaptation service for mobile devices. Proc. 7th Int. Conf. Multimed. Syst. - MMSys '16, pages 1–4, jun 2016. [ DOI | http ]
In most adaptive video streaming systems adaptation decisions rely solely on the available network resources. As the content of a video has a large influence on the perception of quality our belief is that this is not sufficient. Thus, we have proposed a support service for content-aware video adapta- tion on mobile devices: Video Adaptation Service (VAS). Based on the content of a streamed video, the adaptation process is improved by setting a target quality level for a session based on an objective video quality metric. In this work, we demonstrate VAS and its advantages of a reduced data traffic by only streaming the lowest video representa- tion which is necessary to reach a desired quality. By lever- aging the content properties of a video stream, the system is able to keep a stable video quality and at the same time reduce the network load.

[SWT+16] Denny Stohr, Stefan Wilk, Iva Toteva, Wolfgang Effelsberg, and Ralf Steinmetz. Context not Content: A Novel Approach to Real-Time User-Generated Video Composition. In 2016 IEEE Int. Symp. Multimed., page 6. IEEE, 2016.
Instant sharing of user-generated video recordings has become a widely used service on platforms such as YouNow. Yet, it still poses technical challenges, as mobile upload speed and capacities are limited. One proposed solution to address these issues is video composition. It allows switching between multiple video streams–selecting the best source for a given time–for composing a live video of a better overall quality for viewers. Previous approaches require visual analysis of the video streams, usually limiting the scalability of the system. In contrast, our work allows the stream selection to be realized solely on context information, based on video- and service- quality aspects from sensor and network measurements. The implemented monitoring service for context-aware upload of video streams is evaluated in varying network conditions, with diverse user behavior, including camera shaking and user mobility. We show that a higher efficiency for video upload as well as QoE for viewers can be achieved.

[SBD+16] Denny Stohr, Fares Beji, Rahul Dwarakanath, Ralf Steinmetz, and Wolfgang Effelsberg. Mobile NDN-Based Dynamic Adaptive Streaming over HTTP. In IEEE Conf. Local Comput. Networks (LCN), Demo, page 3, Duabi, 2016. IEEE.
Named Data Networking (NDN) is a novel Internet architecture proposing a fundamental alternative to the current host-based routing. It addresses data objects instead of hosts—a concept promises to be especially useful for data-centric appli- cations, e.g., dynamic adaptive streaming over HTTP (DASH). We demonstrate the implications of this networking architecture for the use case of DASH, based on a bare-bone wireless hardware testbed using a custom-build JavaScript player. In our demonstration, we show that NDN provides practical benefits with regard to resilience and for wireless handovers. Further, we propose the developed testbed platform for the practical evaluation of wireless handover and forwarding strategies in NDN networks, including new forms of mobile video production.

[SSW+15] Matthias Schulz, Denny Stohr, Stefan Wilk, Benedikt Rudolph, Wolfgang Effelsberg, and Matthias Hollick. APP and PHY in Harmony: A framework enabling flexible physical layer processing to address application requirements. In Proc. - Int. Conf. Networked Syst. NetSys 2015, 2015. [ DOI ]
Mobile data traffic, particularly mobile video, grows at an unprecedented pace. Despite recent advances at the physical layer, today's wireless network infrastructure cannot keep up with this growth. This is partially due to the missing flexibility to adapt the physical layer continuously to best support both application level as well as network requirements. In this paper we show how to harness the flexibility of advanced physical layers in practice. We designed and implemented a research platform that provides a flexible application-centric physical layer for Android smartphones using software-defined radios (SDRs) as radio interfaces. Our solution allows applications to define flows and apply per-flow settings that are mapped into distinct physical layer settings. As a proof-of-concept and for testbed evaluation, we implemented our system together with a mobile video streaming application. The latter uses a Motion-JPEG based lightweight scalable video codec (SVC) to generate incremental data flows. We show that our system maximizes video quality at the receiver's side, while keeping the energy consumption at the transmitter at a minimum. Our solution demonstrates that jointly optimizing network traffic and application quality is feasible in practice using a flexible physical layer processing approach.

[SSH+15] Denny Stohr, Matthias Schulz, Matthias Hollick, Wolfgang Effelsberg, Hollick Matthias, Wolfgang Effelsberg, Matthias Hollick, and Wolfgang Effelsberg. APP and PHY in Harmony: Demonstrating Scalable Video Streaming Supported by Flexible Physical Layer Control. In Proc. Int. Symp. a World Wireless, Mob. Multimed. Networks, pages 1–3, Boston, USA, jun 2015. IEEE. [ DOI | http ]
Wireless, high-bandwidth data transfer gains increasing significance with a wider adoption of high-quality video streaming on mobile devices. Technologies enabling such data transfer such as 802.11 or LTE can adapt dynamically to changing conditions allowing data transfer in a reliable way. However, applications have little or no influence on how those underlying systems execute such adaptations. Yet, in applications such as video streaming, this could provide important clues on what requirements are relevant for delivery of the data. In this demonstration, we present a system that allows application defined underlay control to use such potentials in a mobile scalable video codec (SVC) based streaming scenario by combining Android smartphones with software-defined radios (SDRs) for advanced PHY layer control.

[SLW+15] Denny Stohr, Tao Li, Stefan Wilk, Silvia Santini, and Wolfgang Effelsberg. An Analysis of the YouNow Live Streaming Platform. In WNM 2015, 9th IEEE Work. Netw. Meas. (WNM 2015), Clearwater, USA, oct 2015.
Video streaming platforms like Twitch.tv or YouNow have attracted the attention of both users and researchers in the last few years. Users increasingly adopt these platforms to share user-generated videos while researchers study their usage patterns to learn how to provide better and new services. In this paper, we focus on the YouNow platform and show the results of a analysis of its traffic patterns and other characteristics. To perform this analysis, we have collected YouNow usage patterns for 85994 users over a period of about one month. Our results show that YouNow's characteristics are in part equal to and in part different from those of other video streaming platforms. Like on YouTube or Twitch.tv, for instance, few YouNow videos attract most of the view requests. On the other side, YouNow sessions are notably shorter than Twitch.tv ones. We believe the observation of these similarities and differences to be crucial to inform the design and implementation of better upcoming video streaming services.

[WSE15] Stefan Wilk, Denny Stohr, and Wolfgang Effelsberg. VAS: A Video Adaptation Service to Support Mobile Video. In 25th Work. Netw. Oper. Syst. Support Digit. Audio Video, mar 2015. [ http ]
Even though cellular networks offer a ubiquitous access to the Internet for mobile devices, their throughput is often insufficient for the rising demand for mobile video. Classical video streaming approaches can not cope with bandwidth fluctuations common in those networks. As a result adaptive approaches for video streaming have been proposed and are increasingly adopted on mobile devices. However, existing adaptive video systems often rely on available network resources alone. As video content properties have a large influence on the perception of occurring quality adaptations our belief is that this is not sufficient. In this work, we thus present a support service for a content-aware video adaptation on mobile devices. Based on the actual video content the adaptation process is improved for both the available network resources and the perception of the user. By leveraging the content properties of a video stream, the system is able to keep a stable video quality and at the same time reduce the network load.

[WRS+15] Stefan Wilk, J Rueckert, Denny Stohr, Richerzhagen B., and Wolfgang Effelsberg. Efficient Video Streaming through Seamless Transitions Between Unicast and Broadcast. In Int. Conf. Networked Syst., Cottbus, Germany, mar 2015. [ http ]
This work describes the prototype of the DFG Collaborative Research Center 1053 on 'Multi Mechanism Adap- tation for the Future Internet' (MAKI) on switching between network mechanisms during runtime. The replacement of whole network mechanisms of a system is called a transition. Transitions are necessary when environmental conditions change significantly. In this demonstration we show the advantages of transitions for a live video streaming. Starting from a client/server based delivery the live video streaming system is proposed which seamlessly adapts to a P2P-based delivery, when the number of concurrent clients significantly increases. As P2P-based unicast- based delivery is inefficient when concurrent accesses come from the same geographic spot, the transition to local dissemination of the live video is investigated. The system identifies a local client which relays the live stream via broadcast to interested local clients. Both the transition from client/server to P2P as well as the replacement of IP Unicast with Broadcast is executed seamlessly for the video player

[SWE14] Denny Stohr, Stefan Wilk, and Wolfgang Effelsberg. Monitoring of User Generated Video Broadcasting Services. In Work. Internet-Scale Multimed. Manag. ACM, 2014.
Mobile video broadcasting services offer users the opportu- nity to instantly share content from their mobile handhelds to a large audience over the Internet. However, existing data caps in cellular network contracts and limitations in their up- load capabilities restrict the adoption of mobile video broad- casting services. Additionally, the quality of those video streams is often reduced by the lack of skills of recording users and the technical limitations of the video capturing devices. Our research focuses on large-scale events that at- tract dozens of users to record video in parallel. In many cases, available network infrastructure is not capable to up- load all video streams in parallel. To make decisions on how to appropriately transmit those video streams, a suitable monitoring of the video generation process is required. For this scenario, a measurement framework is proposed that allows Internet-scale mobile broadcasting services to deliver samples in an optimized way. Our framework architecture analyzes three zones for effectivelymonitoring user-generated video. Besides classical Quality of Service metrics on the net- work state, video quality indicators and additional auxiliary sensor information is gathered. Aim of this framework is an efficient coordination of devices and their uploads based on the currently observed system state. Categories

[RDW+14] Björn Richerzhagen, T U Darmstadt, Stefan Wilk, Julius Rückert, Wolfgang Effelsberg, and Denny Stohr. Transitions in Live Video Streaming Services Categories and Subject Descriptors. In ACM VideoNext 2014, pages 37–38. ACM, dec 2014. [ .pdf ]
This work shows a demonstration on the work of the Col- laborative Research Center 1053 pursuing Multi-Mechanism Adaptation for the Future Internet. Our prototype shows a live video streaming system on mobile devices featuring Peer-to-Peer (P2P) and Client/Server delivery concepts. The focus is to showcase seamless transitions between those mech- anisms, depending on the monitored system state. For this, our system streams video from a central server to mobile devices connected by a WiFi Access Point. The system exe- cutes transitions between client/server delivery and a P2P streaming overlay based on the number of clients streaming the video. For demonstration purposes, transitions can be triggered either automatically or manually. The transitions between both delivery schemes and the clients' states are monitored and visualized on a central UI.

Links:

View Denny Stohr's profile on LinkedIn

View Denny Stohr's profile on ResearchGate
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